With increasing frequency, telephone lines are being used in conjunction with data devices for data communication. Typical data devices in this discussion include computer modems, facsimile machines and any other data equipment which are capable of communicating over a standard two-wire telephone line.
Costs involved in maintaining separate telephone lines for voice and data make it desirable to share a telephone line with both a telephone system, such as a telephone instrument, and a data device. However, problems often result from connecting a data device and a telephone instrument directly to the same telephone line. The most common problem occurs when a data device is using a telephone line to communicate with another similar distant data device, and a telephone instrument on the same telephone line is picked up. The electrical noise generated on the telephone line by the connection of the telephone instrument causes data flow errors between the data devices. The operators of the data equipment may then be required to reset the data devices and restart the data transmissions.
It is, therefore, desirable to have a line control device which will allow only a telephone system or a data device to operate on the same telephone line at one time. This will prevent data flow errors on the telephone line between data devices resulting from the connection and use of a telephone system on that same telephone line.
There are many variations in the electrical characteristics of the telephone circuits, data devices and telephone systems. Typical variations in telephone circuits include (1) an open circuit operating voltage of the telephone line between 15 and 52 volts direct current, (2) a loop resistance of the telephone line between 500 and 4000 ohms and (3) an active, or off-hook, resistance of the telephone system and the data device between 30 and 500 ohms. The control device used must also be able to pass, yet be insensitive to, the high voltage ringing current, usually between 75 and 120 volts alternating current at 16 to 75 hertz. For an automatic line control device to properly detect the active state of a data device or a telephone system on a shared telephone line, the control device must be able to operate over these wide variations of electrical characteristics. The wider the range of electrical variations over which the device can operate, the more universal the compatability of the control device with different telephone lines, data devices and telephone systems.